The structure of the thyroglobulin molecule: By negative staining technique in the electron microscope we have previously demonstrated that thyroglobulin molecules may have either an ovoid or a cylindrical conformation. Cylindrical molecules can be transformed into ovoid molecules by oxidation in vitro. We are now studying the conditions for the transformation of ovoid molecules into cylindrical molecules. We will also study the ultrastructure of thyroglobulin molecules and 12 S subunits isolated from intracellular compartments of thyroid follicle cells in order to elucidate the development of the structure characteristic of thyroglobulin molecules stored in the follicle lumina. Exocytosis and endocytosis in thyroid follicle cells: In our previous in vivo studies we have followed the membrane redistribution involved in TSH-stimulated exocytosis and endocytosis by electron microscopical stereology. We will now extend these studies by following in vitro the redistribution of labeled membrane in isolated thyroid follicles.